What does Greencast mean?
A denim dye process in which the garment is dyed with green sulfur and then indigo to give it a slight bluish green look over time.
Heddels explains Greencast
Greencast denim is denim that has been dyed with a green sulfur dye before being dyed with indigo. This effect becomes more apparent as the jeans are worn and the indigo dye fades to expose the green dye underneath.
Here is an example of greencast denim: